Pipes A and B can fill a cistern in 15 hours together. But if
these pipes operate separately A takes 40 hours less than B
to fill the tank. In how many hours the pipe A will fill the
cistern working alone?

Options

A. 60
B. 20
C. 40
D. 15

Answer

(b)

Explanation

Let A takes x hours, then ` B = (x + 40)` hours
`1/x + 1/(x+40) =1/15`
Solve, `x = 20`